1. Field of the Invention
The present disclosure relates generally to techniques for signal transmission with antenna diversity and has been developed with particular but not exclusive attention paid to the possible application in the framework of telecommunications systems based upon the CDMA/3GPP (Code-division Multiple Access/Third Generation Partnership Project) standard in its various versions, for example.
Reference to this possible application must not, however, be interpreted as in any way limiting the scope of the invention.
2. Description of the Related Art
In order to increase the performance of the aforesaid telecommunication systems, there have been proposed various transmission schemes: in this connection, the 3GPP standard has defined both open-loop techniques, referred to, respectively, as STTD and TSTD, and closed-loop solutions, based upon beam-forming techniques.
In order to improve the performance of the system, the 3GPP standard contemplates the use of techniques based upon the use of two transmitting antennas set at the base stations (BTS) in combination with strategies for encoding the data transmitted by them.
Recourse to the principle of antenna diversity in transmission, and, in particular, to the approach referred to as space-time coding (STC) with a number of transmitting antennas greater than two and increasingly complex encodings, draw on the pioneering results reported by G. J. Foschini et al. in Bell Labs Tech. J., Autumn 1996, and in the works of Telatar, “Capacity of multiantenna Gaussian channels” AT&T Bell Labs, Tech. Rep., June 1995 and once again of Foschini and Gans in Wireless Personal Comm., March 1998.
The above studies have demonstrated that the spectral efficiency of a device can be considerably increased by adopting diversity techniques, not only in reception, but also in transmission. Space-time coding (STC) techniques are able to exploit the characteristics of multiple-reflection transmission environments to distinguish independent signallings transmitted simultaneously in the same frequency band. These techniques prove very effective in environments (such as, precisely, the environment of mobile communication networks), in which the main problem to be faced is that of multipath fading.
In particular, Space-Time Transmit Diversity (STTD) techniques, to which reference has already been made previously, is a type of space-time coding that enables improvement of the performance in terms of error probability by maintaining unvaried the transmission rate by means of a pair of antennas in transmission and a corresponding encoding of the data flow sent to them. In view of its simplicity, this solution has been introduced in the 3G standard in the implementation stage.
The essential characteristics of this solution adopted by the 3GPP/UMTS standard may be inferred from the diagram of FIG. 1. This scheme for data encoding, which is applicable in the cellular-communication environment in so far as it functions also with just one antenna in reception, basically envisages that the sequence of the input bits (b0, b1, b2, b3) is transmitted unaltered via a first antenna A and is, instead, subjected to a combined action of shuffling and of complementing that is such as to bring the sequence of four bits referred to previously to be sent for transmission via the second antenna in the form of the modified sequence (b2, b3, b0, b1).
From the point of view of QPSK coding and its representation in complex notation, this operation on the bits is mapped in a conjugation if the second bit (LSB) of the pair is complemented or in a conjugation with phase reversal (i.e., multiplication by −1) in the case where it is the first bit (MSB) of the pair that is complemented.
To complete the picture of the currently available solutions, it is also possible to cite the technique known as BLAST (Bell Labs Layered Space-Time), which contemplates the use of more than one antenna both in transmission and in reception. With this technique, spectral efficiencies higher than 30 bits/sec/Hz have been obtained, which cannot be obtained with conventional detection schemes, in environments that are not very noisy or not noisy at all and affected by multiple reflections.
Also a solution known as V-BLAST (Vertical BLAST) can be cited, which is substantially based upon a scheme that is simplified as compared to the BLAST technique, such as not to require codings between the flows transmitted and such as to enable, albeit with a presumably lower complexity, a performance in terms of spectral efficiency that is comparable with that of the BLAST technique.
At the moment, there are being studied techniques that envisage further improvement of the performance of the system by increasing the number of antennas in transmission and by partially modifying encoding, albeit by maintaining the compatibility with respect to the preceding versions of the 3GPP/UMTS standard—Release 1999.
For example, in the document RP020130 (now TR25.869) entitled “Tx diversity solutions for multipath antennas” presented at the TSG-RAN Meeting No. 15 held on Mar. 5-8, 2002, there is proposed the solution represented in FIG. 2.
This is, in practice, a scheme that contemplates the presence of four antennas or, more precisely, four pseudo-antennas designated, respectively, by Aa, Ab, Ac and Ad. By adopting said scheme, the input signal x(t) is subjected, in a block designated by S, to the STTD-Rel. '99 coding procedure for each pair of antennas. This procedure uses the technique also known as Alamouti space-time block coding for generating two distinct signals x1 and x2, which are to be subjected first to a multiplication by respective factors χ and ξ in two multipliers in view of the supply to the antennas Aa and Ac. The same signals are once again subjected to a multiplication by two factors ejφ and ejψ, respectively, (in practice, a phase rotation is performed) in view of the supply to the antennas Ab and Ad.
In practice, the pseudo-antennas in question are defined, respectively, as:Aa=A1+A2,Ab=A3+A4,Ac=A1−A2, andAd=A3−A4,in the case where a balancing of power is required between the transmitting antennas; otherwise, we have:Aa=A1,Ab=A2,Ac=A3, andAd=A4,where A1, A2, A3 and A4 are the physical antennas.
The diagram represented in FIG. 2 uses the Alamouti technique, which is based upon the concept of transmitting the first branch with diversity according to the STTD scheme (s1, S2, . . . ) via a first antenna (A1) and a replica subjected to phase rotation via the second antenna (A2). The second branch with STTD diversity is transmitted in a similar way via the antennas A3 and A4.
Once again, FIG. 3 illustrates schematically a technique referred to as “phase hopping”, which contemplates a phase rotation between the antennas and between the symbols according to a given sequence of values (by maintaining the phase constant for at least two consecutive symbols).
In particular, the phase patterns proposed for the pseudo-antenna 2 and for the pseudo-antenna 4 are respectively: {0, 135, 270, 45, 180, 315, 90, 225} and {180, 315, 90, 225, 0, 135, 270, 45}, i.e., φ=ψ+π. Of course, the values indicated in braces refer to angles expressed in degrees.